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The invention described in this patent was made by an employee of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties.
This invention relates to screening apparatus particularly suited for screening articles or objects such as assembly line parts and baggage.
Considering first the baggage aspect, during the last ten years patents directed to luggage inspecting apparatus began to appear in the patent art. Recently there has been a significant increase in patents relating to luggage scanning, each with a different approach. Illustrating these approaches, the invention that is the subject of U.S. Pat. No. 5,247,561 is based on Compton scattering. Compton scattering is proportional to mass density. Energy such as X-rays applied to luggage containers is measured, and a three-dimensional image of the objects in the container is constructed. In U.S. Pat. No. 6,304,629 a conveyor belt passes through an isolating tunnel which is a substantially enclosed area of the tunnel that is sealed-off by curtains so that no X-rays will leak out. The X-ray baggage system disclosed in U.S. Pat. No. 6,088,423 uses three parallel stationary beams of X-rays in an enclosed tunnel. The parallel rays are spaced from each other in the tunnel along the direction in which baggage moves through the tunnel. The three fan beams produce sets of scan lines from which X-ray attenuation data are measured. The background section of that patent refers to various patents such as U.S. Pat. No. 5,796,802, U.S. Pat. No. 6,256,404, and U.S. Pat. No. 943,388 directed to tomographically produced X-ray images. Computed X-ray tomography is a technique which has been in use in medical applications for more than 20 years. In U.S. Pat. No. 5,796,802, in order to save additional time required in computed tomography, the data are first only partially analyzed. From this pre-selected, pre-screened, data it is determined whether a physical attribute of interest is present. In U.S. Pat. No. 5,943,388 a plurality of X-ray detectors, each having an output which includes at least two threshold data levels, are utilized. Display means responsive to the plurality of X-ray detectors produce tomographic images for different energy bands. In the computed tomography system of U.S. Pat. No. 6,256,404 the computed tomography machine scans the field of view to generate scan data. Using the size and location of the scanned object two portions of pixels in the field of view are identified. Only pixels that provide information related to the scanned object, the first portion of pixels, are processed during image reconstruction.
A study of the different detection technologies in use today reveals that although the use of a linear detector array allows a high inspection speed, its output signal does not provide sufficiently detailed information. A coherent X-ray scatter spectra analysis enables a better detection and much lower false alarm rate ( less than 1%), but it is more time consuming, leading to a lower throughput. Dual energy detection systems base their decision on an estimation of the density of objects in a bag or other container. This is done by a combined evaluation of two different X-ray images generated at two different X-ray voltages (e.g. 150 kV and 75 kV). The approach involves dedicated image processing to separate different objects superimposed on one another in the projected image. The measured densities are compared with library values of densities of known explosives. This multi-view technique uses two X-ray systems with two different views (bottom view, top view). The estimated material density values generated from the two-projection image pixel data are compared with the typical density data for identifying image objects.
It can be seen that the prior art illustrates the conclusion reached in U.S. Pat. No. 6,088,423 that existing baggage inspection systems meet some but not all of the inspection needs. Thus, three dimensional technologies are utilized but only as tomographic and dual energy techniques. Absent from the variety of approaches discussed is stereoscopic imaging technology. Rather than relying on shades or textures to discover the shape of an object, a more effective way would be to use two or more images of the same object each taken from a slightly different perspective or viewpoint. If there is too much separation or deviation of the X-ray tubes the image will look flat., or the result may be a depth perception which is exaggerated or reduced. Indeed, the stereo pairs may not fuse at all, with the viewer seeing two separate images. Yet with a slight separation interference occurs, possibly explaining why stereoscopic systems have not been employed.
Stereo pairs create three dimensional images when binocular disparity cues are correct. Binocular disparity depends primarily on both the distance of the X-ray source to the projection plane or target, and the separation of the right and left X-ray devices or tubes. In the case of X-ray scanning, when the binocular disparity is within correct limits, the X-rays from the two X-ray tubes converge at the target. This convergence results in cross exposure interference, to be discussed hereinafter in more detail, leading to blurred images. This is an undesirable condition which, as pointed out, may have been the barrier to the stereo pairs approach to object scanning. Rather, when multiple X-ray sources have been used they have been orthogonal beam X-ray systems in which X-ray beams were projected at right angles toward the top and a side of an object.
An object of this invention is to provide a system for scanning articles such as luggage and other objects by the production of stereo pairs. Assuming that the articles are traveling from left to right, this is accomplished by the use of left and right X-ray tubes aligned on a common line in the same plane above the path, and by left and right X-ray sensors disposed below the path to sense X-rays emitted by the tubes. An appropriate, or desired, left viewpoint or perspective angle is first established, determined by the angle formed by the common line and a line to the target. Next, a right perspective angle is computed using the distance from the left X-ray tube to the target, the distance from the left X-ray tube to an imaginary point, and the distance from that imaginary point to the target. The imaginary point is a point which is within the stereoscopic range on the common line between the tubes. The computed or determined right perspective angle, then, is a right viewpoint from a slightly different perspective leading to a stereo pair. Using these left and right perspectives, or perspective angles, the left and right X-ray sources or tubes can then be spaced away from each other an actual distance such that the stereo base is greater than a normal stereo base required for a stereo pair of images. This spacing eliminates cross exposure interference while still obtaining X-ray exposures from two slightly different perspectives. Since the articles are traveling on a conveyor belt, or otherwise, from left to right the two images required for stereoscopic vision will have been sensed in sequence, although they are out of order. To this end a sensor is used to determine luggage belt positions further enabling an estimate of the coordinates of the luggage belt appearing in the center-of-view of the left and right sensors. A processor then separately stores radiographic data for objects appearing within the field-of-view of each sensor along with the belt coordinates of the center-of-view for each sensor, respectively. The processor provides a continuous sequence of stereo images for stereographic display by retrieving left and right radiographic images with similar belt coordinate estimates for transmission to the stereo display device. A human operator upon viewing the stereo radiographic pairs may control the belt motion and display sequence to more closely examine or re-examine items through interaction with the operator control interface panel.
Normal stereoscopic vision depends essentially on the fact that each eye sees from a slightly different perspective. The key to producing a stereoscopic display, then, is getting a different perspective to each eye. Parallax is merely the apparent displacement of a viewed point that results from a change in the point of observation or from relocation from one viewing spot to another. The brain uses the differing parallax to determine distances to objects being viewed. This same effect lies at the heart of viewing stereo pairs taken from two lateral positions. When the stereo pair is properly positioned left-to-right and then viewed through a stereoscopic device the eyes send a signal to the brain, which on further processing, creates a perception of depth. It is this perception of depth that must be obtained herein to yield a three dimensional effect.
As indicated previously, the degree of the stereo effect depends on both the distance of the X-ray source to the projection plane or article, and the separation of the left and right X-ray sources. Too large a separation is very difficult to resolve and is known as hyperstereo. It is this separation of left and right X-ray sources which is of concern herein. However, in the case of X-ray devices required for scanning a separation less than the hyperstereo distance is too close. To illustrate this FIG. 1 is given. Referring to that figure, X-ray sources 1 and 2 are illustrated, along with objects 4, on conveyor belt 6. A sensor or detector 7 is disposed within conveyor 6. It can be seen that the rays leading from two properly spaced X-ray sources 1 and 2 cross each other, the result being cross exposure interference producing an undesirable result. For stereo image pairs the images must be constrained not to overlap. By the practice of this invention stereo pairs can be used to obtain three dimensional views of articles being scanned. For a better understanding of how this is accomplished a specific embodiment of the invention will now be described in conjunction with the accompanying drawings.